1. Field of the Invention
The present invention relates generally to earth boring bits of both the fixed cutter and the rolling cutter variety. More specifically, the present invention relates to the cutting structures and cutting elements of such earth boring bits.
2. Description of the Prior Art
Commercially available earth boring bits can generally be divided into the rolling cutter bits, having either steel teeth or tungsten carbide inserts and fixed cutter or drag bits. Modern fixed cutter bits typically utilize either natural diamonds or artificial or man-made diamonds as cutting elements. The diamond containing fixed bits can be generally classified as either steel bodied bits or matrix bits. The steel bodied bits are machined from a steel block and typically have cutting elements which are press-fit into openings provided in the bit face. The matrix bit is formed by coating a hollow tubular steel mandrel in a casting mold with metal bonded hard material, such as tungsten carbide. The casting mold is of a configuration which will give a bit of the desired form. In the past, the cutting elements were typically either polycrystalline diamond compact (PDC) cutters braised within an opening provided in the matrix backing or are thermally stable polycrystalline diamond cutters which are cast within recesses provided in the matrix backing.
The rolling cutter bit employs at least one rolling cone cutter, rotatably mounted thereon. As with the fixed or drag bit, the rolling cutter bit is secured to the lower end of a drill string that is rotated from the surface of the earth. The cutters mounted on the bit roll and slide upon the bottom of the borehole as the drill string is rotated, thereby engaging and disintegrating the formation material.
Despite their generally similar overall function, fixed bits and rolling cutter bits are subjected to different operative forces which dictate fundamental design differences. For example, in the case of rolling cutter bits, the cutters roll and slide along the bottom of the borehole. The cutters, and the shafts on which they are rotatably mounted, are thus subjected to large static loads from the weight on the bit, and large transient or shock loads encountered as the cutters roll and slide along the uneven surface of the bottom of the borehole. Thus, earth boring bits of the rolling cutter variety are typically provided with precision formed journal bearings and bearing surfaces, as well as sealed lubrication systems to increase the drilling life of the bits. The lubrication systems typically are sealed to avoid lubricant lose and to prevent contamination of the bearings by foreign matter such as abrasive particles encountered in the borehole. A pressure compensator system minimizes pressure differential across the seal so that lubricant pressure is equal to or slightly greater than the hydrostatic pressure in the annular space between the bit and the sidewall of the borehole. These features would not normally be present in the fixed cutter or drag bit.
Super-hard materials, including natural and synthetic diamond materials, have been used in fixed cutter or drag type bits for many years. Recently, there has been a general effort to introduce the improved material properties of natural and synthetic diamond type materials into earth boring bits of the rolling cutter variety, as well. However, differences in the forces exerted upon the cutting elements of fixed cutter bits versus bits of the rolling cutter variety come into play. Fixed cutter bits employ the shearing mode of disintegration of the earthen formation almost exclusively. Although diamond and other super-hard materials possess excellent hardness and other material properties, they are generally considered too brittle for most cutting element applications in rolling cutter bits, with an exception being the shear cutting gage insert of such bits. The gage cutters, located on the corner and sidewall of the cutter are subjected to crushing and scraping or shearing actions, while the borehole wall is produced in a pure sliding and scraping (shearing) mode. In the corner and on the sidewall of the borehole, the cutting elements have to do most of the work and are subjected to extreme stresses, which makes them prone to breakdown prematurely and/or wear rapidly.
Recent attempts to introduce diamond and similar materials into rolling cutter bits have relied on a diamond layer or table secured to a substrate or backing material of fracture-tough hard metal, usually cemented tungsten carbide. The substrate is thought to supplement the diamond or super-hard material with its increased toughness, resulting in a cutting element with satisfactory hardness and toughness which diamond alone is not thought to provide.
In addition to the problem of brittleness, diamond inserts of the above general type have presented additional problems, such as the tendency of the diamond or super-hard material to delaminate from the substrate. Several attempts have been made to increase the strength of the interface. U.S. Pat. No. 4,604,106, to Hall et al., discloses a transition layer interface that gradually transitions between the properties of the super-hard material and the substrate material at the interface between them to resist delamination. U.S. Pat. No. 5,544,713, to Dennis, uses an interrupted interface on the metal carbide stud to reduce spalling. U.S. Pat. No. 5,351,772, to Smith, provides a non-planar interface between the diamond table and the substrate. U.S. Pat. No. 5,355,969, to Hardy et al. is another example of a non-planar interface between a super-hard material and the substrate in a PDC drill bit.
Thus, many of the prior art attempts to incorporate diamond or other super-hard materials into the cutting structures of earth boring bits have presented design problems which compromised the overall performance characteristics of the bits.
A need exists, therefore, for earth boring bits having super-hard cutting elements that are relatively easy to manufacture with a satisfactory combination of material properties.
A need also exists for an earth boring bit having wear surfaces, such as the cutting surfaces and cutting elements, with improved properties to extend the useful life of the bit.
Another object of the invention is to provide a earth boring bit having diamond reinforced wear surfaces which surfaces are less brittle and are less likely to delaminate from their substrate than were the prior art materials.
A need also exists for an earth boring bit having cutting elements with a lower coefficient of friction formed by finer diamond starting materials and possessing smoother surfaces than cutting elements of the prior art.